1. Field of the Invention
The present invention relates to a method of initializing a phase transformation-type optical disk capable of overwriting data, and more particularly, to a method of initializing a phase transformation-type optical disk capable of improving jitter characteristics.
2. Description of the Related Art
In general, an optical disk is classified into a read only optical disk such as a CD-ROM, a rewritable optical disk capable of overwriting information, and a write once read many (WORM) optical disk capable of recording information once.
As the rewritable optical disk, a phase transformation-type optical disk has been widely used, which records information using the phase transformation of its recording layer. As shown in FIG. 1, the phase transformation-type optical disk 10 includes a first dielectric layer 2, a recording layer 3, a second dielectric layer 4 and a reflective layer 5 which are sequentially stacked on a substrate 1. The recording layer 3 is deposited in an amorphous phase. That is, as shown in FIG. 2A, the recording layer 3 has an amorphous portion A in which a small amount of crystalline grains (C) is distributed. For recording information in the recording layer 3, the recording layer 3 must be transformed from the amorphous phase to a crystalline phase by irradiating laser beams, which is called "initialization" of an optical disk.
That is, through the initialization, the state of the recording layer 3 is changed from the amorphous phase which has a reflective index that is relatively low, to the crystalline phase which has a reflective index that is relatively high.
In a conventional initialization method of an optical disk, low power of laser beams of 3.about.6 mW is irradiated onto the amorphous recording layer 3 to crystallize the amorphous recording layer 3. Then, a high power of laser beams, e.g., 8.about.13 mW, which is enough to heat the recording layer 3 above the melting point thereof, is irradiated onto the crystalline recording layer 3, to change the crystalline phase into the amorphous phase having a lower reflective index, thereby recording information. That is, information is written or erased using the difference in reflective indices between the crystalline and amorphous phases.
However, in the initialization of the recording layer 3, as shown in FIG. 2B, the amorphous portion A is changed into a crystalline portion A' containing small grains, and the small crystalline grains (C) grow to coarse crystalline grains C'. According to experimentation, it was observed that the crystals C having a diameter of 9.about.10.ANG., distributed in the amorphous portion A of the deposited recording layer, grew to grains having a diameter of approximately 13.ANG.after initialization. Thus, after the initialization process, the small grains and coarse grains coexist, resulting in an inhomogeneous structure. Accordingly, jitter error increases. Here, jitter error represents deviation between the length of an information signal to be recorded in the recording layer 3 and the length of the actual recorded information signal. As such deviation increases, the playback characteristics deteriorate. As shown by the dotted lines in FIG. 5, the jitter error increases until the information is written and erased one to five times on the same position of the recording layer, thereby lowering reliability of the phase transformation-type optical disk. Meanwhile, the jitter error is relatively low after 5 write and erase processes. This is because the structure of the recording layer, which was not uniform in the initial state, gradually becomes homogeneous by repeated heating using the laser beams.
According to a second conventional method of initializing a phase transformation-type optical disk, a recording layer of an optical disk is crystallized by irradiating high power laser beams, e.g., 250.about.700 mW, onto the recording layer. However, as shown by the dot-and-dashed line of FIG. 5, jitter error increases over 1.about.10 rewriting processes. In particular, jitter error is the highest after two rewritings. Also, in the initialization process of an optical disk according to the second conventional method, crystals coexisting within the amorphous recording layer grow to make the structure uneven, thereby causing errors. Thus, a method of initializing a phase transformation-type optical disk, capable of suppressing the jitter errors which are quite high when several rewritings are performed at the initial stage, has been required.